Individuals who suffer from a balance control disorder are abnormally prone to falling and have poor gait control when walking or engaging in other movement tasks. A balance control disorder may be the result of a wide variety of sensory and/or motor disorders that impair the posture and equilibrium control of the subject. Balance control disorders may be caused by vestibular, proprioceptive, central nervous system (CNS) or other defects. Vestibular defects are abnormalities related to the part of the auditory nerve in the inner ear that carries sensory information related to body equilibrium. Proprioceptive defects are abnormalities in the stimuli provided to or received by a subject's muscles to maintain equilibrium control. Balance control disorders may affect both sides (left and right) of a subject, or may be manifested as a greater instability in one direction.
In order to make a correct assessment of a subject's balance control, and thereby to take remedial measures, an examining physician, or physical therapist, may determine the subject's balance control ability for a number of motor tasks, such as standing, getting up out of a chair, walking down steps, etc. By observing the subject performing such motor tasks, the physician may be able to determine if the subject's balance control is within normal limits and, if not, how best to bring balance control near or within normal limits again. However, to provide a more accurate and objective assessment of the individual's sensory and motor components of posture and equilibrium, a test system which provides an objective quantifiable assessment of balance control is required.
Quantitative information on the human sense of balance can be obtained using a variety of methods and devices. Quantitative information on the efficacy of the human sense of balance can be obtained by the electrophysiological measurement of eye movements or of the postural responses of the limbs. A balance control deficit is indicated if a response is outside of the limits expected for individuals having a normal balance function. Quantitative postural information may also be obtained by measuring contractile activity of the muscles generating the internal body forces for maintaining the equilibrium position using electromyographic (EMG) recordings.
Balance control defects are often quantified by recording body sway, i.e., the displacement of the body from the equilibrium position. Quantification of the postural sway of a subject is known as "stabilometry" or "posturography". One method for quantifying balance disorders involves the measurement of body sway in terms of displacement of the center of foot pressure (CFP), sometimes termed "center of force", generated by the inherent instability of a test subject standing on a fixed support surface. CFP is computed from the signals provided by force transducers which are typically embedded in the four corners of the support surface. The force transducer outputs are employed to obtain a projection, on the support surface platform, of the resultant forces acting at the subject's center of gravity. An anterior-posterior, front-to-back, projection is obtained by assuming that the difference between the force detected by the fore and aft force transducer-pairs equals torque about the ankle joint. The anterior-posterior projection is obtained by dividing the ankle torque by the total vertical force. This calculation assumes that the upright body can be represented by a simple upright pendulum. Thus, only the effect of movement at the ankle joint is considered, the effect of movements at the knee and hip joints is ignored. A similar calculation employs the signals provided by the lateral pairs of force transducers, on each side of the support platform, to obtain a lateral force projection. The vectorial sum of the anterior-posterior and lateral force projections equals the CFP. As body sway frequencies exceed 0.2 Hz, however, this method for estimating the movement of the body's center of gravity based on CFP becomes increasingly inaccurate, because oscillations of the upper body enter the CFP measurements as inertial reaction forces. Furthermore, if the multi-link nature of the body is ignored, serious errors in understanding a subject's balance disorders can occur.
Investigators have used different types of force platforms to analyze postural sway. Some such force platforms are specifically targeted toward tests for analyzing balance disorders caused by vestibular defects. Quantitative examination of CFP data suggests that subjects having a unilateral vestibular balance deficit, e.g., a balance deficit caused solely by impairment of the vestibular end organs in the inner ear, perform within normal ranges when tests are employed using a fixed force sensitive support surface to perform the balance tests. For this reason, techniques have been introduced which make the control of spontaneous sway by a subject positioned on the CFP measuring support surface more difficult. These techniques make quantification of a vestibular balance disorder easier, by interrupting the non-vestibular sensory inputs that the subject may otherwise use to maintain his balance.
One such technique involves moving the support surface so that it is tilted (forward or backward) in relation to changes in the subject's CFP. This type of controlled platform instability may be obtained using a purely mechanical device, or with a more flexible electronic and computer controlled motor unit. The movement of the support surface platform disrupts the somatosensory inputs which would otherwise be available to the subject. A second technique involves the use of a moveable visual surround, which surrounds the subject, and which is moved to follow the subject's body sway, as estimated by CFP measurement of the subject. This technique disrupts the visual stabilization inputs used by the subject to maintain balance control. By disrupting the somatosensory and visual inputs, a test procedure for analyzing a subject's balance control is able to focus more particularly on the vestibular balance control mechanism. Examples of such test systems and procedures are described in more detail in U.S. Pat. Nos. 4,738,269, 5,052,406 and 5,303,715, issued to Nashner, et al.
Analysis of tests employing these methods of CFP sway quantification have indicated that destabilization of a support surface beneath a subject provides a major diagnostic improvement. However, the diagnostic sensitivity of such methods is still limited, because other variables like trunk rotation and EMG response amplitudes are not taken into account. Furthermore, such methods cannot be used to examine the directional sensitivity (left or right) of a balance disorder, because the movements of the support surface platform are limited to only one axis of rotation (forward and backward). Destabilizing a visual surround by moving it in relation to the CFP provides little additional diagnostic information as far as a vestibular balance deficit is concerned. Furthermore, the destabilization of the visual surround is also limited to forward-backward movements.
Another system that may be used to measure body sway employs light-weight light-emitting sources that are mounted on a subject's body. However, such three-dimensional camera based systems are typically prohibitively expensive for most physical therapy practices specializing in rehabilitation of gait and balance deficits. Moreover, these systems also have a number of technical drawbacks, including excessive computer power requirements, limited on-line capabilities, sensitivity to interfering light sources, and limited range of operation. Thus, although such systems are capable of quantifying gait and other dynamic postural abnormalities, which cannot be achieved using CFP measuring support surfaces, this advantage is outweighed by the price and ease-of-operation advantages of more conventional CFP systems.
A more advanced method for performing non-invasive, sensitive, and reliable tests for the presence of abnormalities in the postural sway of a human subject employs light-weight wearable body sway sensors, such as velocity transducers, that are attached to the upper body of a subject. The body sway sensor output signals are transformed into detailed angular displacement and velocity information by a microprocessor based system processor. The body sway information provided by the body sway sensors is not limited in accuracy by the assumptions used for calculating body sway based on CFP from the signals provided by force transducers embedded in a force plate support surface. The angular position and velocity body sway information derived from the signals provided by the body sway sensors is presented in useful information formats that are displayed to an opera-or on an operator's display unit. The operator's display may provide for comparisons between body sway information obtained from different examination trials or between body sway information obtained from examination trials and body sway information obtained from a normal sample population. The operator's display may also provide an objective measure of the subject's stability. Thus, a physician or physical therapist is able to make an accurate diagnosis of the subject's balance and gait deficits from the body sway information provided on the operator display.
After a balance deficit has been diagnosed and quantified, a physician may prescribe remedial measures to bring the subject's balance control near or within normal limits. The physician may prescribe medication that reduces the action of peripheral senses on the brain. Alteratively, the physician may prescribe a course of physical therapy, which will typically last at least several weeks, with the object of training the subject's brain to deal with a reduced sense of balance when trying to maintain the body upright and prevent a fall. However, neither of these techniques will have an immediate rehabilitory effect on the subject's balance deficit. Moreover, medication can have side effects, and can also reduce the capability of the brain to process balance information from the peripheral senses. A course of physical therapy requires a long training period which may extend over more than two months. These difficulties and limitations associated with conventional remedial measures for dealing with balance deficits are most problematic when the subject is older and likely to have a falling tendency.
Rehabilitation of a subject's balance control deficit may also be accomplished by providing rehabilitory postural feedback to the subject. Such postural feedback may be provided, for example, based upon body sway angle and sway velocity information obtained from body sway sensors, such as velocity transducers, which are attached to the subject's body. The feedback may be in the form of visual, auditory, or tactile stimulation, or may be provided in the form of an electrical signal that is used to directly stimulate the vestibular nerve. A single type of feedback may be used, or different types of feedback may be provided to a subject in combination. A visual feedback system may be incorporated in a pair of light-weight eyewear. An imaging system mounted on the eyewear is used to project a visual feedback display into the eye of the wearer of the eyewear. Auditory feedback may be provided using audio headphones, or a similar device, and frequency modulations around different audible tone center frequencies. Tactile feedback may be provided by vibrators placed on a subject that are used to convey a sense of rotation of the subject's torso. These types of rehabilitory feedback immediately augment the balance signals normally used by the subject's brain to help stabilize body sway and improve balance.